Brian, the example that Channing provided is referred to as a French lock by toolmakers and machinist. After looking at the picture closely I believe the lock screw was added later to correct a customer complaint of a loose tang screw. The lock screw head does not match the engraving on the Tang screw head and the lock screw head does not appear to be flush with the tang. The serial number is not in a range that an engineering change would be expected to take place.
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